Re Adding a ribbon tweeter
Then you would no longer be a genuine 604 listener, for what its worth I would get or borrow another newer 604 crossover, yours are aged and the caps are dried out & maybe leaky,do a comparison test using pink noise source & you'll hear a high end difference. GP has a HF horn mod, which might help and they also sell a new 604 crossovers,better than the type you have, main thing is it ALTEC. (Todd White should be able to advise)